The present invention is generally related to thermal-chemical polishing devices and methods suitable for diamond films, and more particularly to a high-speed and low-cost thermo-chemical polishing device and method for polishing diamond films.
The technology in growing and depositing diamond films on substrates is constantly improving and has reached a level of maturity where diamond films are commonly used in different manufacturing industries. The deposition methods include PVD (physical vapor deposition) which generally forms a thinner layer of diamond film and different types of CVD (chemical vapor deposition) which generally form a thicker layer of diamond film. CVD depositions are divided into MPCVD, RF plasma CVD, HF CVD and thermal CVD which can under low pressure form a diamond film with a thickness up to several hundred micrometers on the surface of a workpiece. Due to its superior mechanical/optical properties such as high strength, low rate of wear, good heat conductivity, anti-radiation, high rate of electron transmission, and high corrosion resistance, the diamond film has infused into various industries creating a revolution of new applications.
A diamond-like carbon (DLC) formed by CVD for example can be used to form cutting tools (surface plating) or grinding powder due to its ultra high strength and good heat dissipating properties which can increase the cutting performance and at the same time extending its life span. Diamond films can also be applied to the surface of molds for protection which assists in heat dissipation (with an increase in the rate of heat dissipation 4 times that of silver) to allow better flow and release of the mold, as a result the life span of the mold increases between 2 to 10 times. Furthermore special properties of diamond films such as high electron transmission, high heat tolerance, anti-acid, and anti-radiation allow usage under extreme conditions such as in high temperature engines, and radar equipment. More importantly, diamond has been recently used as a semiconductor material where diamond chips have a transmission speed double that of silicon chips and yet are not handicapped by the operating temperature limit of 150xc2x0 C. In MEMS components, diamond can eliminate the wear problem of silicon and can be used in high rpm components such as micro motors. At the same time, diamond has played an important role in the medical and optical field. Diamond is a suitable material to be implanted into a human body because of the high stiffness, low wear, and anti-corrosion properties. In the field of optics, diamond can produce highly transparent viewfinders, lenses, planar displays, and scanners. The introduction of diamond has lead to an increase in quality in various applications.
Regardless of the characteristic requirement of diamond for various applications, the level of surface planarity and smoothness has to meet stringent standard to prevent degradation of the superior properties of diamond such as high strength, low wear factor, and high transparency and retain the original performance of the material. Therefore in relation, the technique and technology in diamond polishing is the key to whether the use of diamond can be widely applicable in manufacturing. As commonly known, diamond has the highest hardness among all materials and therefore is usually used as a cutting tool or a polishing pad to polish other materials. In contrast, for diamond itself to be polished to reach planarity is very difficult. Furthermore diamond is a chemically inert material and is the only material that will not react with acid (also not with aqua regia) under 600xc2x0 C. so it is extremely difficult to process diamond. In addition, the low wear and low coefficient of expansion contribute to the difficulty in processing diamond. Please refer to FIG. 5 showing an enlarged diagram of a workpiece 50 having a diamond film 50a being in contact with a polishing pad 55 during polishing. From the figure, due to the difference in the coefficient of expansion between the diamond film 50a and the substrate 50b, the higher coefficient of expansion of the substrate 50b will cause the workpiece 50 to experience warpage which prevents the planar polishing surface of the workpiece 50 from being perfectly in touch with the polishing pad 55. As a result, the quality of the polishing is unsatisfactory. The quality of the diamond polishing also depends on the film thickness, the substrate material, the forming temperature, and the polishing temperature which all affects the consistency of the quality.
Apart from warpage caused by differences in CTE (coefficient of thermal expansion), significant variation in film thickness may be introduced by the inhomogeneous distribution of plasma field (as shown in FIG. 5). In this aspect, a good level of planarity cannot be achieved without removing a large amount of material but contrarily removing a large amount of material requires a huge amount of time which is not time efficient. Consequently the technology for a diamond polishing device that allows high removal rate is in demand.
Diamond powder or diamond wheels are currently used to polish diamond films. However, the removal rate is extremely slow and the cost is high. Prior art uses a method by diffusion between diamond and transition metal (such as Fe) or rare-earth element (such as Se) at high temperature (above 500xc2x0 C.) or by graphitization. The polishing pad is made of Fe or Nixe2x80x94Se and the thermo-chemical polishing is performed at a temperature between 500xc2x0 C. and 950xc2x0 C. in an oxygen or hydrogen environment. FIGS. 6 and 6A show a conventional polishing machine 2. A heating device 61 is attached on a polishing pad 60 for increasing the temperature during polishing. A holder 62 is provided to secure a workpiece 63 by orienting the surface of the diamond film against the polishing pad 60 for performing thermal-chemical polishing. The polishing method used in prior art relies on the expensive and heavy heating device 61 which increases the cost of the polishing machine 2 and also at the same time decreases the performance of polishing due to the limitation on rotational speed of the polishing pad 60 from the added weight. For diamond films under 100 xcexcm, it may take up to approximately 100 hours for the surface to achieve planarity together with extensive repair and replacement; therefore it is impractical for volume production and unsatisfactory to the industries. Furthermore, the conventional polishing machine 2 needs to be operated in a vacuum environment where after polishing a recover process is performed so an air tight mask 64 is provided on the periphery of the polishing pad 60. This design increases the cost of production and maintenance and also affects the rotational speed of the polishing pad 60. This conventional polishing machine 2 is uneconomical and cannot ameliorate the problem of warpage that is shown in FIG. 5 in order to maintain quality consistency.
Therefore the design of a thermal-chemical polishing machine that has a high polishing rate and at the same time low cost and easy to maintain which can satisfy economies of scale is the most focused area of research and development.
An object of the present invention is to provide a thermal-chemical polishing machine that has a high rate of polishing.
Another object of the present invention is to provide a thermal-chemical polishing machine that is low in cost.
Another object of the present invention is to provide a thermal-chemical polishing machine that is easy to repair and replace.
In meeting the aforementioned objects, the present invention provides a thermal-chemical polishing device and method thereof, comprising: a high rpm first rotatable shaft with a circumferential surface having a material that can react with diamond at high temperature and having a predefined heating region; a high rpm rotatable and translational second shaft which is coupled in perpendicular to the first rotatable shaft; and a heating unit that is used to heat up the heating region on the circumferential surface of the first shaft. The second shaft engages with the first shaft to allow the workpiece having the diamond film in the holder to make contact with the heating region. As the diamond film of the workpiece makes contact with the material on the circumferential surface of the first shaft, a chemical reaction occurs for performing a thermal-chemical polish.
The present invention provides a method of thermal-chemical polishing used in the above-mentioned thermal-chemical device, the method comprising the following steps: initiate high rpm rotation of the first shaft; heat up the heating region on the circumferential surface of the first shaft by the heating unit; initiate high rpm rotation of the second shaft and move in the direction of the axis of rotation to allow the diamond film of the workpiece to make contact with the predefined heating region on the first shaft for performing a thermal-chemical polishing. During the polishing process, the second shaft moves axially in a suitable feed-in rate to maintain the surface of the diamond film in appropriate contact with the heating region and to terminate the polishing process as long as the diamond film reaches a level of accepted planarity.
The material of the first shaft that provides the chemical reaction with the diamond is a transition metal (such as Fe) or a rare-earth element (such as Se), which reduces the material cost and increases rotational speed by making the first shaft into a hollow shaft or plating with a transition metal or rare-earth elements onto the surface of the first shaft which can made of a lighter material. Moreover the heating device can be an infrared type or an inductance type which can heat up the predefined heating region to above 450xc2x0 C. for allowing a chemical reaction to take place between the diamond and the transition metals or the rare-earth elements; the second shaft moves back and forth along a direction perpendicular to the first shaft during polishing to allow sliding contact with the heating region of the first shaft to prevent the thickness of one particular area of the surface in the predefined heating region eroding too quickly.
Furthermore a sensing device is located on the second shaft for detecting the positive cutting force exerted or the surface thickness of the circumferential surface of the predefined heating region during polishing. The sensing device adjusts the feed-in according to the result of the detection so the level of contact between the surface of the diamond film and the predefined heating region is maintained at an appropriate level for controlling the quality and efficiency of the polishing process.
According to the above, the present invention provides a thermal-chemical polishing device and a method thereof which uses special devices to greatly increase the rate of polishing. At the same time, the number of components decreases so no extra weight is added to the polishing shaft that causes a decrease in the rotational speed. Furthermore the cost is lowered because there is no wastage of material and lastly repair and maintenance are easy to improve efficiency.